1. Field of the Invention
The present invention relates to a temperature-compensated crystal oscillator mounted in communications equipment such as a cellular phone, and the like.
2. Description of the Related Art
A temperature-compensated crystal oscillator mounted in communications equipment comprises a crystal oscillation circuit incorporating an AT-cut crystal resonator (unit) in a frequency band of 10 MHz as the oscillation source thereof, and a temperature-compensated circuit using a frequency adjustment circuit for adjusting an oscillation frequency of the crystal oscillation circuit so as to stabilize the oscillation frequency by canceling out the cubic curve temperature characteristic of the AT-cut crystal resonator.
Since the temperature-compensated crystal oscillator is mounted in portable communications equipment, there is a requirement that its power consumption is low. However, the temperature-compensated crystal oscillator is specified such that the power source thereof is to be kept turned off during most of times when standing by for phone calls, and consequently, even a product thereof with a power source at 3V, having current consumption of not less than 1 mA, has been permissible in the past.
As there has recently been seen a mounting trend of adopting the CDMA (code division multiple access) system aiming at international sharing of a common communications system, and under the specification of the CDMA system, the temperature-compensated crystal oscillator is required not to turn the power source thereof off even during periods of standing by for phone calls, there has been an increasing demand for further lower power consumption.
Now, the most effective method of achieving lower power consumption is to lower a driving voltage of the crystal oscillation circuit.
Accordingly, for a temperature-compensated crystal oscillator aiming at lower power consumption, a configuration is adopted wherein a temperature-compensated crystal oscillation circuit is driven by a constant voltage generation circuit (a voltage regulator).
FIG. 5 shows a configuration of such a conventional temperature-compensated crystal oscillator by way of example. With this type of temperature-compensated crystal oscillator, a temperature-compensated crystal oscillation circuit 1 is driven by a constant voltage generation circuit 3.
The constant voltage generation circuit 3 comprises a differential circuit 5 supplied with a reference voltage A as one of the inputs, a dc load 7, a driver 9 by use of a FET for driving the dc load 7 and the temperature-compensated crystal oscillation circuit 1 under control of the differential circuit 5, and a phase-compensation capacitor 11 for prevention of self-oscillation.
The dc load 7 has a function of supplying feedback signals to the differential circuit 5, and determining a ratio of the reference voltage A to a driving voltage for the temperature-compensated crystal oscillation circuit 1, that is, an amplification factor. In this example, the dc load 7 comprises a first resistance 7a and a second resistance 7b that are identical in kind and connected in series.
The differential circuit 5 shown in FIG. 5 is made up of CMOS FETs using either an n-type substrate or a p-type substrate, however, it can also be made up of bipolar transistors.
The dc load 7 is not an essential constituent element of the constant voltage generation circuit 3 shown in FIG. 5. If the reference voltage A equivalent to the driving voltage for the temperature-compensated crystal oscillation circuit 1 is available, the dc load 7 may be dispensed with, and an output of he driver 9 may be supplied straight to the differential circuit 5 as the feedback signals thereto.
In any case, lower power consumption of the temperature-compensated crystal oscillator can be achieved by rendering the diving voltage for the temperature-compensated crystal oscillation circuit 1 lower than a source voltage V1-V2 (normally, V2=ground potential) with the use of the constant voltage generation circuit 3.
However, in the case where the temperature-compensated crystal oscillation circuit 1 is driven by use of the constant voltage generation circuit 3 in this way, there will arise a problem that phase noise increases in comparison with a case where the temperature-compensated crystal oscillation circuit 1 is driven directly by the source voltage.
The reason for this is described hereinafter.
The constant voltage generation circuit 3 of the temperature-compensated crystal oscillator is provided with the driver 9 having a small on-resistance, and performs feedback control such that an output voltage thereof is kept at the reference voltage A multiplied by a constant factor. And the feedback control is performed by use of the differential circuit 5.
The feedback control is a prerequisite for the constant voltage generation circuit 3, because a constant voltage can be outputted by the feedback control even in the case of manufacturing variation of device characteristics, variation in the source voltage, variation in load, and so forth.
Meanwhile, the temperature-compensated crystal oscillation circuit 1 is a stable load on a long term basis, however, has a characteristic of current thereof pulsating on a shorter term basis, depending on a phase condition of oscillation. That is, it is a load having a property of its impedance undergoing changes on a shorter term basis.
If such a variable load on a short term basis is driven by the conventional constant voltage generation circuit 3, the variable load is contained in feedback loop, so that signals fed back to the differential circuit 5 are caused to pulsate.
Consequently, the constant voltage generation circuit 3 performs an operation so as to cancel out variation in the load in order to keep the output voltage thereof at a constant level. If the constant voltage generation circuit 3 has a response speed fast enough to be able to follow variation in the load at a frequency of the temperature-compensated crystal oscillation circuit 1, the output voltage can be maintained at the constant level.
However, in the constant voltage generation circuit 3, it is extremely difficult to operate the differential circuit 5 controlling the driver 9 at such a high speed enabling the same to follow the variation in the load of the temperature-compensated crystal oscillation circuit 1 in the frequency band of 10 MHz.
Accordingly, in response to the variation in the frequency band of 10 MHz of the temperature-compensated crystal oscillation circuit 1, the constant voltage generation circuit 3 performs an operation so as to compensate for the variation in the load at a frequency (for example, in the order of 1 kHz) lower than that for the former.
More specifically, the variation in the load proceeds at a speed higher than a speed at which the constant voltage generation circuit 3 operates to maintain the output voltage thereof at the constant level. Hence the output voltage can not be maintained at the constant level displaying a characteristic of undergoing variation at a frequency lower than that in the frequency band of 10 MHz.
Since an oscillation frequency of the temperature-compensated crystal oscillation circuit 1 varies depending on the driving voltage, the oscillation frequency of the temperature-compensated crystal oscillation circuit 1 undergoes fine modulation when the output voltage of the constant voltage generation circuit 3 varies at a certain frequency. The fine modulation of the oscillation frequency will show up in the form of deterioration in phase noise.
That is, the cause for the deterioration in phase noise occurring when the temperature-compensated crystal oscillation circuit 1 is driven by the constant voltage generation circuit 3 is that short term variation in the load proceeding at a high speed is fed back to the differential circuit 5.